1. Field of the Invention
This invention relates generally to a piston diaphragm pump for the delivery of liquids, and more particularly surfactants, wetting agents or the like, in doses.
2. Discussion of Related Art
A piston diaphragm pump of the above type is known from DE-OS 28 31 437. In this known piston diaphragm pump, the particular liquid is drawn into the suction chamber through the flexible diaphragm. The liquid to be dosed is displaced from the pressure chamber solely by the pump piston which is fixedly connected to the diaphragm and which, to this end, is sealingly guided in the pump tube surrounding the pressure chamber. Not only does this require corresponding sealing, it can also lead to seizing of the pump piston after prolonged breaks in operation through drying of the liquid to be dosed. Since, in addition, the pump piston has a much smaller effective cross-section than the pump diaphragm and since both operate with the same lift because they are fixedly connected at both ends, less liquid is always displaced from the pump pressure chamber during the particular delivery stroke of the pump than is taken into the suction chamber through the diaphragm. The result of this is that the suction chamber has to be connected to the liquid reservoir by a special liquid line designed to be shut off by a non-return valve to enable the quantity of liquid delivered in excess into the suction chamber to be returned to the reservoir during the return stroke of the pump membrane. Another disadvantage of the known pump is that, because of its comparatively small delivery cross-section, the pump piston and, hence, the diaphragm have to complete comparatively large strokes to enable a certain volume of liquid to be delivered in dosed form.
3. Summary of the Invention
Accordingly, the problem addressed by the present invention is to improve and complete a piston diaphragm pump of the type mentioned above so that it does not have any of the disadvantages mentioned above and, instead, operates with shorter pump strokes for delivering comparable quantities of liquid, without requiring sealed guiding of the pump piston in the pump tube, thereby preventing seizing of the piston after prolonged breaks in operation. The modified piston diaphragm pump of the invention includes a working chamber lying on the side of the pump diaphragm remote from the suction chamber serving as an additional pump pressure chamber which is permanently connected to a longitudinal bore through a piston leading to the pump tube pressure chamber by transverse channels present in the pump piston behind the piston valve. This ensures that the pumping movements of the diaphragm also become fully effective on the pressure chamber side with the pump piston serving as a drive element for the pump diaphragm designed to be actuated by an electromagnetic coil with a return spring acting thereon. Accordingly, the piston does not have to be specially sealed in the pump tube as in the prior art, but instead may be guided therein with greater tolerances, so that there is virtually no danger of seizing through liquid which has dried out after prolonged breaks in operation. Since, in addition, the liquid to be displaced from the pressure chamber via the pump diaphragm corresponds in volume to the particular quantity of liquid drawn into the suction chamber, there is no need for a special return pipe from the suction chamber to the liquid reservoir, as in the prior art. Comparable delivery volumes of liquid can be obtained with relatively small delivery strokes of the diaphragm and, hence, the pump piston which in turn leads--in the same way as its unsealed guiding in the pump tube--to less heating and to reduced friction losses relative to the prior pump.
The piston valve is preferably provided in the immediate vicinity of that end of the pump piston which passes through the pump diaphragm, while the transverse channels are preferably provided immediately behind that end of the pump piston. This provides for a particularly compact construction of that part of the pump situated on the pressure chamber side, particularly if, in addition, the pump housing is tapered in the manner of a funnel towards the pump tube in its region surrounding the transverse channels and the diaphragm pressure chamber.
In another embodiment of the invention, a pump piston stop provided with a seal is advantageously provided in that part of the housing which surrounds the pump suction chamber, acting as a safety valve separating the pressure chamber from the suction chamber in the stop position of the pump piston brought about by the return spring. In this manner, unwanted flow of the delivery medium from the pressure chamber into the suction chamber and vice versa is effectively prevented, even after prolonged periods of stoppage of the pump when the piston valve may possibly be prevented from closing by dry, crystallized liquid residues.
Finally, in another embodiment of the invention, a rectifier which converts the current supplying the magnetic coil into pulsating direct current is built into the current lead and is preferably integrated into that part of the pump housing which surrounds the electromagnetic coil. This provides for quiet, current-saving driving of the diaphragm pump through the pump piston acting as an armature. The switching on and off of the electromagnetic diaphragm drive is generally controlled by a pulse generator which is installed in the consumer pipe or container to be charged with the liquid to be dosed, and which responds to the particular demand for that liquid.